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although； at present； neither elephants nor hippopotamuses occur
in America。 Fifthly； in the middle of the continents； in regions
most remote from the sea； we find an infinite number of shells；
of which the most part belong to animals of those kinds which
still exist in southern seas； but of which many others have no
living analogues； so that these species appear to be lost；
destroyed by some unknown cause。 It is needless to inquire how
far these statements are strictly accurate； they are
sufficiently so to justify Buffon's conclusions that the dry
land was once beneath the sea； that the formation of the
fossiliferous rocks must have occupied a vastly greater lapse of
time than that traditionally ascribed to the age of the earth；
that fossil remains indicate different climatal conditions to
have obtained in former times； and especially that the polar
regions were once warmer； that many species of animals and
plants have become extinct； and that geological change has had
something to do with geographical distribution。
But these propositions almost constitute the frame…work of
palaeontology。 In order to complete it but one addition was
needed； and that was made； in the last years of the eighteenth
century； by William Smith； whose work comes so near our own
times that many living men may have been personally acquainted
with him。 This modest land…surveyor； whose business took him
into many parts of England； profited by the peculiarly
favourable conditions offered by the arrangement of our
secondary strata to make a careful examination and comparison of
their fossil contents at different points of the large area over
which they extend。 The result of his accurate and widely…
extended observations was to establish the important truth that
each stratum contains certain fossils which are peculiar to it；
and that the order in which the strata； characterised by these
fossils； are super…imposed one upon the other is always the
same。 This most important generalisation was rapidly verified
and extended to all parts of the world accessible to geologists；
and now it rests upon such an immense mass of observations as to
be one of the best established truths of natural science。 To the
geologist the discovery was of infinite importance as it enabled
him to identify rocks of the same relative age； however their
continuity might be interrupted or their composition altered。
But to the biologist it had a still deeper meaning； for it
demonstrated that； throughout the prodigious duration of time
registered by the fossiliferous rocks； the living population of
the earth had undergone continual changes； not merely by the
extinction of a certain number of the species which had at first
existed； but by the continual generation of new species； and the
no less constant extinction of old ones。
Thus the broad outlines of palaeontology； in so far as it is the
common property of both the geologist and the biologist； were
marked out at the close of the last century。 In tracing its
subsequent progress I must confine myself to the province of
biology； and； indeed； to the influence of palaeontology upon
zoological morphology。 And I accept this limitation the more
willingly as the no less important topic of the bearing of
geology and of palaeontology upon distribution has been
luminously treated in the address of the President of the
Geographical Section。
The succession of the species of animals and plants in time
being established； the first question which the zoologist or the
botanist had to ask himself was； What is the relation of these
successive species one to another？ And it is a curious
circumstance that the most important event in the history of
palaeontology which immediately succeeded William Smith's
generalisation was a discovery which； could it have been rightly
appreciated at the time； would have gone far towards suggesting
the answer； which was in fact delayed for more than half a
century。 I refer to Cuvier's investigation of the mammalian
fossils yielded by the quarries in the older tertiary rocks of
Montmartre； among the chief results of which was the bringing to
light of two genera of extinct hoofed quadrupeds； the
Anoplotherium and the Palaeotherium。 The rich
materials at Cuvier's disposition enabled him to obtain a full
knowledge of the osteology and of the dentition of these two
forms； and consequently to compare their structure critically
with that of existing hoofed animals。 The effect of this
comparison was to prove that the Anoplotherium； though it
presented many points of resemblance with the pigs on the one
hand and with the ruminants on the other； differed from both to
such an extent that it could find a place in neither group。
In fact； it held； in some respects； an intermediate position；
tending to bridge over the interval between these two groups；
which in the existing fauna are so distinct。 In the same way；
the Palaeotherium tended to connect forms so different as
the tapir； the rhinoceros； and the horse。 Subsequent
investigations have brought to light a variety of facts of the
same order； the most curious and striking of which are those
which prove the existence； in the mesozoic epoch； of a series of
forms intermediate between birds and reptilestwo classes of
vertebrate animals which at present appear to be more widely
separated than any others。 Yet the interval between them is
completely filled； in the mesozoic fauna； by birds which have
reptilian characters； on the one side； and reptiles which have
ornithic characters； on the other。 So again； while the group of
fishes； termed ganoids； is； at the present time； so distinct
from that of the dipnoi； or mudfishes； that they have been
reckoned as distinct orders； the Devonian strata present us with
forms of which it is impossible to say with certainty whether
they are dipnoi or whether they are ganoids。
Agassiz's long and elaborate researches upon fossil fishes；
published between 1833 and 1842； led him to suggest the
existence of another kind of relation between ancient and modern
forms of life。 He observed that the oldest fishes present
many characters which recall the embryonic conditions of
existing fishes； and that； not only among fishes； but in several
groups of the invertebrata which have a long palaeontological
history； the latest forms are more modified； more specialised；
than the earlier。 The fact that the dentition of the older
tertiary ungulate and carnivorous mammals is always complete；
noticed by Professor Owen； illustrated the same generalisation。
Another no less suggestive observation was made by Mr。 Darwin；
whose personal investigations during the voyage of the
Beagle led him to remark upon the singular fact； that the
fauna； which immediately precedes that at present existing in
any geographical province of distribution； presents the same
peculiarities as its successor。 Thus； in South America and in
Australia； the later tertiary or quaternary fossils show that
the fauna which immediately preceded that of the present day
was； in the one case； as much characterised by edentates and； in
the other； by marsupials as it is now； although the species of
the older are largely different from those of the newer fauna。
However clearly these indications might point in one direction；
the question of the exact relation of the successive forms of
animal and vegetable life could be satisfactorily settled only
in one way； namely； by comparing； stage by stage； the series of
forms presented by one and the same type throughout a long
space of time。 Within the last few years this has been done
fully in the case of the horse； less completely in the case of
the other principal types of the ungulata and of the carnivora；
and all these investigations tend to one general result； namely；
that； in any given series； the successive members of that series
present a gradually increasing specialisation of structure。
That is to say； if any such mammal at present existing has
specially modified and reduced limbs or dentition and
complicated brain； its predecessors in time show less and less
modification and reduction in limbs and teeth and a less highly
developed brain。 The labours of Gaudry； Marsh； and Cope furnish
abundant illustrations of this law from the marvellous fossil
wealth of Pikermi and the vast uninterrupted series of tertiary
rocks in the territories of North America。
I will now sum up the results of this sketch of the rise and
progress of palaeontology。 The whole fabric of palaeontology is
based upon two propositions： the first is； that fossils are the
remains of animals and plants； and the second is； that the
stratified rocks in which they are found are sedimentary
deposits； and each of these propositions is founded upon the
same axiom； that like effects imply like causes。 If there is any
cause competent to pro